Conventionally, the method for producing trifluoromethanesulfonic anhydride by reacting trifluoromethanesulfonic acid with phosphorus pentoxide, as illustrated in the formula below, has been known (see Patent Documents 1 and 2). In this method, ordinarily an excess amount of phosphorus pentoxide (2 to 4 times mole) is used with respect to the trifluoromethanesulfonic acid in order to increase the conversion ratio of the trifluoromethanesulfonic acid.6CF3SO3H+P2O5→3(CF3SO2)2O+2H3PO4 
This conventional production method, however, has the problem that when the reaction rate of the trifluoromethanesulfonic acid approaches approximately 60%, glass-like polyphosphoric acid produced as a byproduct accumulates and hardens the reaction solution, making it difficult to stir the reaction solution, and the reaction cannot be carried out further. It is also not easy to remove the byproduct of polyphosphoric acid or carry out processing after the reaction, and it is extremely difficult to distill and recover the unreacted trifluoromethanesulfonic acid and to dilute the polyphosphoric acid. For these reasons, the industrial production of trifluoromethanesulfonic anhydride has been plagued with problems.
In the conventional production method, since a large amount of unreacted trifluoromethanesulfonic acid remains, there is known the method of adding water or phosphoric acid, or adding further trifluoromethanesulfonate metal salt, to the distillation residue and then distilling this to recover the unreacted trifluoromethanesulfonic acid (see Patent Document 1). However, since the reaction between the solidified distillation residue and water is accompanied by a sudden rise in temperature due to the reaction heat, it is dangerous and difficult to perform industrially.
A method of using a fluorine-based solvent in order to improve the reduced yield due to hardening of the reaction solution is known (Patent Documents 3 and 4). Examples of the solvents that may be used in this method include fluoroalkylsulfonic anhydrides, fluoroalkylsulfonate esters, perfluoroalkanes, perfluroalkylamines, and perfluoropolyethers that contain the desired substance, but none of these solvents sufficiently dissolves polyphosphoric acid. As a result, after the reaction, it is necessary to add water or phosphoric acid as in the case of the conventional method in order to sufficiently recover the produced trifluoromethanesulfonic anhydride by distillation.
Another method that is known is that of producing trifluoromethanesulfonic anhydride by reacting with phosphorous pentachloride, phosphorous trichloride, and chlorine (Patent Documents 5 and 6). In this case, however, there is the problem that when, after the reaction, the produced trifluoromethanesulfonic anhydride is recovered by distillation, a byproduct of phosphorus oxychloride and the trifluoromethanesulfonic anhydride are difficult to separate because their boiling points are close to one another.
Patent Document 1: U.S. Pat. No. 5,004,829
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H02-268148
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. H09-227498
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. H10-114734
Patent Document 5: Japanese Unexamined Patent Application, First Publication No. H11-236365
Patent Document 6: Japanese Unexamined Patent Application, First Publication No. H11-236366